Okay... it appears as if having multiboot installed and upgrading kernel power causes the problem. Specifically, multiboot specifies

2.6.28.10power40

the upgrade to kernel power removes that and multiboot can't boot.

In my case, the multiboot option 0 which should do a stock kernel has also stopped working, so I suspect I need to reflash my rootfs.

However, if people have other ideas, let me know.